Heater basics in a '77

I took my car out today and it was nice out but very cold @22deg.F. I drove for about 20 minutes and was never able to get heat. My coolant temp never went beyond the first dash mark. Is there a thermostat that needs to open before the heater will work? It didn't work on the trip home either. Last time I drove the car it was @45deg and the heat worked great. When I arrived at my destination, I opened thehood and the radiator was warm/hot to the touch but definitely not too hot. The coolant lines to the heater boxes were warm.

 

the radiator was hot, just not too hot to touch. I even opened the bleed screw and there was a little air in the system until the hot coolant steamed out.